Hanoi appears to be preparing a response to the recent escalation of attacks by guerrillas resisting the Vietnamese occupation of Kampuchea (Cambodia). The unexpected return home for consultations of Vietnam's ambassador to Bangkok may be the first sign that the Vietnamese are concerned about the upsurge in activity, mainly by the Khmer Rouge troops of former Kampuchean prime minister Pol Pot.
The departure of Ambassador Tran Quang Co coincided with the absence from his post of his counterpart in the Kampuchean capital of Phnom Penh, Ngo Dien. Both followed closely after the first spate of reported attacks.
Ngo Dien has returned to Phnom Penh, but Mr. Co is still in Hanoi. The Thai-Kampuchean border is quiet, but usually well-informed sources say Hanoi will respond to the attacks ''in kind.''
In the last month the Khmer Rouge claim to have attacked, and in some cases temporarily occupied, four provincial capitals - the cities of Siem Reap, Kompong Thom, Pursat, and Battambang. Most observers accept that the Khmer Rouge have been unusually active but doubt the magnitude of their claims.
As usual, journalists trying to discover what really happened suffer from several handicaps. First is access: This correspondent is rarely allowed inside either Vietnam or Kampuchea.
Second is the Khmer Rouge habit of grossly exaggerating their reports. Each year they claim to control most of the country and to have destroyed the equivalent of whole divisions of the Vietnamese army, while foreign visitors and residents report little fighting and relative normality.
Third is the will on the part of Vietnam's adversaries to believe at least part of the Khmer Rouge claims, in order to buttress their own belief that Vietnam is in deep trouble in Kampuchea.
Even so, something definitely happened in the northern city of Siem Reap.
The most detailed non-Khmer Rouge report says that on the night of Jan. 26-27 , Khmer Rouge guerrillas penetrated the eastern suburbs of the city and attacked a Vietnamese command post, killing about 25 Vietnamese troops.
All sources, including those close to Hanoi, agree that some sort of attack was made on the east of the city. All agree that the guerrillas came from Phnom (mount) Kulen 30 miles northeast of the city - once best known as an important religious and historical site, now a Khmer Rouge guerrilla base.
But the forces of Prince Sihanouk, one of the Khmer Rouges' nominal coalition partners, suggest that in reality the Khmer Rouge did not penetrate the city - they simply rocketed it.
The Sihanoukists add that they attacked an area northwest of the city the same night, apparently to divert Vietnamese attention. They say that for the first time this attack was coordinated with the Khmer Rouge - an act that may make them liable to Vietnamese retaliation.
An interesting feature of the incident has been admission by the United States, apparently for the first time, that its satellites overfly Kampuchea. The US Embassy here has apparently shown friendly missions satellite photographs of Siem Reap after the attack. One person who says he saw the photos, however, says they show mostly smoke rising from the eastern edge of the city.
The other attacks are even harder to substantiate. In most cases something probably happened, but much less than claimed by the Khmer Rouge.
On Feb. 15, for example, three days after an alleged Khmer Rouge attack on Battambang, a United Nations team visited the city. They found nothing amiss, and an Indian diplomat accompanying the team was even able to make a spur-of-the-moment trip outside the city without any visible security measures.
There are at least two possible reasons for the increased Khmer Rouge activity.
First, the guerrillas reportedly received an unusually large amount of supplies from China late last year. (The Chinese say the resupply was ''only average.'') Peking likes to see its aid produce results and may have urged the Khmer Rouge to do something spectacular.
Meanwhile the two noncommunist coalition members have been gradually overcoming some of the differences that separated them. Both talk publicly of the need to achieve military parity with the Khmer Rouge; privately they hint they would like to swamp them. The Khmer Rouge may have decided it was time to remind their partners that they are still the most effective military force.